The quality (i.e. the dryness) of wet
steam can be found by using a separating and throttling calorimeter.
Figure 2.6.4 shows the general arrangement of the device.

The separator, as its name suggests,
physically separates the water droplets from the steam sample. This
alone would give us a good idea of the dryness of the steam, despite
that the separation is not complete, because, as we have seen the
dryness fraction is the ratio of the mass of pure steam to the total
mass of the steam.

Having separated out the water droplets
we can find their mass to give us the mass of water in the sample,
m1. The ‘pure steam’ is then condensed to allow its mass to be
found, m2. Then,

Dryness fraction from separator, x =
m2/ (m1 + m2)

A more accurate answer is obtained by
connecting the outlet from the separator directly to a throttle and
finding the dryness fraction of the partly dried steam.

In the throttling calorimeter, the
steam issuing through the orifice must be superheated, or we have two
dryness fractions, neither of which we can find. Throttling improves
the quality of the steam, which is already high after passing through
the separator, therefore superheated steam at this point is not
difficult to create.

To find the enthalpy of the superheated
steam, we need its temperature and its pressure.

For the throttling calorimeter,

Enthalpy before = enthalpy after
throttling

hf + x.hfg = enthalpy from superheat
tables

If we call the dryness from the
separator, x1, and the dryness from the throttling calorimeter x2,
the dryness fraction of the steam sample is x, given by, x = x1 × x2